In addition to the infantile lethal form of glycogen storage disease with cardiomyopathy (GSD Type IIa, Pompe disease) 1,4 glucosidase or acid maltase deficiency has been reported in a few children and adults (GSD Type IIb or IIc) erroneously thought to have muscular dystrophies. The clinical heterogeneity of the muscle involvement in these latter cases is illustrated in a 12-year-old boy presenting with a right lumbar mass, growth retardation, muscular weakness including difficulty in walking, and marked elevations of muscle and liver enzymes. Light- and electron-microscopic examination of specimens from the lumbar mass, apparently normal skeletal muscle and liver, showed typical changes consistent with the biochemical and enzymatic features of acid maltase deficiency. GSD Type IIb and IIc are more frequent than suspected, may present as local pseudohypertrophy and should be considered in patients with progressive muscle disease and abnormal serum enzymes.